Growth Behavior Of ?-phase In Ti-55531 Alloy And Its Effect On Tensile Fracture

Ti-55531(Ti-5Mo-5Cr-5V-3Al-1Zr)is a new near-? titanium alloy with the high tensile strength,good fracture toughness and good hardenability.Heat treatment strengthening of Ti-55531 alloy depend on the aging decomposition of ? metastable phase formed by quenching,it can be improved by precipitation of ? phase.The mechanical property mainly depends on the volume fraction,morphology,size and distribution of ? phases.However,the precipitation rate of ? phase from metastable ? matrix is fast and the size is small during aging.Based on this,the precipitation and growth behavior of ? phase in Ti-55531 alloy was systematically studied in this paper,and the effect of ? morphology on tensile fracture of the alloy was also discussed.Firstly,the Ti-55531 alloy was heated by direct resistance for short time with self-designed electric heating fixture.The morphology of ? phase at different growth stages was observed by rapid cooling after aging treatment in different ways.Secondly,the tensile properties of the samples treated by different heat treatment processes were tested,and the effects of?-morphology on tensile fracture of the alloys were studied.From the above studies,the following conclusions are drawn:There are three forms of ?-phase precipitated from ?-crystal during aging treatment at high temperature: triangle,cluster and dendrite.After aging 30 min treatment,? phase will be completely covered with ? grains.In the process of fracture,the dislocations will accumulate in the interior of ?-phase or at the ?/? interface of the triangle shape,and the cracks will be initiated in the stress concentration.The ability of cluster ? phase to block dislocation movement is very limited,which makes both deformation and dislocation gather at ? grain boundary,thus forming crack extending along crystal,and dendriform ? phase is easy to induce local hardening and form microcrack in its interior.The precipitation behavior of ?-assisted ? phase and its effect on tensile property were investigated by two step aging heat treatments.A large number of isothermal ? phases were precipitated by low-temperature aging processes(300 °C ~ 400 °C).After the subsequent high temperature aging processes(580°C),super refined ? precipitations was precipitated under ?assisted,showing two types of morphologies: laths and colonies.The tensile strength of 1620 MPa is improved apparently by a large number of fine ? phase precipitation.Fracture mechanism shows that brittle fracture easily propagates along the ? grain boundary with colonies,and microvoids form at laths boundaries.The in-situ tensile analysis shows that there are a large number of slip lines and subcrystalline boundaries in the microstructure of ? grain structure during the tensile process.During the tensile fracture process,the microcracks nucleation mainly at the slip line and grain boundary.The ? precipitation hinders the movement of the dislocation,so that the dislocation is pinned to the inside of the ? phase.It improves the strength and plasticity of the alloy.However,it is easy to form ? colonies near grain boundary,which leads to local hardening,resulting in the formation of microcracks at the bound of the ? colonies.It is the main reason for reducing the plasticity of alloy.